1. Field of the Invention:
The invention relates to a process for determining the thickness and/or change in thickness of a thin layer applied to a carrier, in which the carrier coated with the thin layer is radiated with a plane polarized light beam and the reflectance on the carrier and layer is detected, after which the measured amount of detected light is converted into a layer thickness and/or change in layer thickness. The invention also relates to the use of this process for determining protein/carrier and protein/protein interactions, as well as to a means for carrying out such a process.
2. Description of the Related Art:
A process as described above and application thereof for quantification of immunological reactions on surfaces is known from an article in Analytical Biochemistry 145 (1985), pp. 106-112. In this well-known method, light polarized in the plane of incidence at an angle virtually equal to the (pseudo) Brewster angle, is directed to the carrier. The intensity of the reflected light is measured. With the build-up of a thin layer on the carrier, the reflection value of the reflectance will change, and hence so will the intensity. Thus, the change in intensity forms a measure of the thickness of the thin layer.
To the known process there is the problem of its being susceptible to fluctuations in intensity of the light source used, because a change in intensity is used as a direct measure of (a change in) thickness. Although this source of error may be eliminated to a considerable extent by e.g. the use of reliable laser light sources such as a helium-neon laser, which provides a high intensity at a stable wave length, the intensity dependence of the process does lead to restrictions in this way, considering that a helium-neon laser is not necessarily universally applicable. Also, in the well-known method of calculating the measuring sensitivity, both the intensity of the light beam and the reflection and transmission properties of all optical components in the optical path will have to be known.
Processes for determining layer thicknesses in which the measuring results do not directly vary with the intensity of the light reflected by the substrate are known in themselves from a great many publications. Those processes, however, are always particular embodiments of ellipsometry determinations. The drawback to such processes is that for the determination of every layer thickness two measurements are required which are carried out at separate moments at different settings of the analyzer. In IBM J. Res. Develop. of November 1973, pp. 472-489 it has been suggested that the analyzer be rotated, so that the time interval between the two measurements can be considerably reduced. But there still remains the drawback of requiring a computer program for the determination of the layer thickness.
U.S. patent specification No. 2,666,355 describes a continuous determination of the layer thickness using a conventional ellipsometer. Although in the determination by it of the growth of a protein layer, different measuring settings are not required for each determination and the intensity of the reflected light appears to be a measure of the thickness of the thin layer, such a method offers hardly any advantages, if at all, over the aforementioned method discussed in Analytical Biochemistry.
Finally, mention is made of an article by Smith et al. in Solid State Electronics. Vol. 12, No. 10, October 1967, Pergamon Press, pp. 765-774. The article mentions a polarizing beam splitter for the separation of p- and s-polarized beam which is totally different from that envisaged by the process of the present invention. For, the known process has for its object to readily obtain two light beams of equal intensity merely by setting the analyzer. By applying the same procedure to incident light and rotating the detector assembler through 45.degree., the ellipticity parameters .DELTA. and .PSI. can be determined from the different angular position of the analyzer. So it is not possible for this process to be used for continuous determination of the layer thickness.